Welcome! the cat is adorable. as you know, everyone at pixelation is a mean jerk who exists solely to provide harsh but constructive criticism for our members. in honor of your first post, here are a few things to help get you started!
1 - painting on a white background is a guaranteed way to end up with really skewed colors in your final piece (unless this is your intention)
2 - if you use outlines with broad color fills, your work will probably look more like a coloring book than a painting
3 - when you do use lines, focus on keeping them smooth, as opposed to jagged
4 - pixel art follows all the same rules as regular art, most of the time. this means lighting, volumes, perspective, anatomy, all that stuff still counts!
5 - this certainly varies from artist to artist, but i find that using a tablet instead of a mouse to do pixel art (even the cheapest crappiest tablet on EARTH) made a very big difference in my art. it's not a magical skill enabler, but it does break down some of the mouse barriers, and makes it easier to sketch or paint the way you would with traditional media, rather than clicking about like its a spreadsheet app.
6 - something that helped me a LOT when i first started (and that still helps me) and something that has I think helped a few other artists around here is to start your pixel art by working with just a very few colors at first, all grayscale even. like white, black, and 3 shades of gray. and sketch out your whole picture that way, in detail. painters do this all the time, they call it a "value study", and what it does is allows you to pinpoint structure, shading, and line problems before you've gone and used 10 different colors to carefully shade your shapes (before they are ready!). so instead of finishing your piece and going "oh crap this is the wrong size" you will notice it when you've only used 4 colors instead, and its very to change things and experiment.
I hope that helps! looking forward to seeing your progress